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Abstract

Background

Impaired glucose tolerance (IGT) and high body mass index (BMI) are recognized risk
factors for type 2 diabetes mellitus (T2DM). However, data suggest that also underweight
predisposes people to develop T2DM. Here, we experimentally tested if already moderate
underweight is associated with impaired glucose tolerance as compared to normal weight
controls. Obese subjects were included as additional reference group.

Conclusion

Our study demonstrates that not only obese but also healthy people with moderate underweight
display glucose intolerance. It is therefore suggested that all deviations from normal
BMI may be accompanied by an increased risk of developing T2DM in later life indicating
that the maintenance of body weight within the normal range has first priority in
the prevention of this disease.

Background

Type 2 diabetes mellitus is a global epidemic. A considerable body of evidence indicates
that impaired glucose tolerance and body mass index represent risk factors for developing
T2DM. To date, the association between BMI and impaired glucose tolerance is well
documented in case of overweight and obesity [1] but little is known about the relationship between low weight and IGT. However, studies
show a linear relationship between low birth weight and the prevalence of T2DM in
adult life [2]. Moreover, IGT has been observed in severe malnutrition [3], certain forms of cancer [4], and anorexia nervosa [5], i.e. diseases accompanied by extremely low BMI within the scope of a pathologic
state. These findings indicate that, apart from obesity, also a BMI < 20 kg/m2 may be associated with IGT and therefore with an increased risk of T2DM.

Against this background, we hypothesized that, in analogy to obese subjects, also
healthy individuals with moderate underweight display IGT. In order to test this hypothesis,
we investigated whether glucose disposal rates, giving key information about glucose
tolerance, and calculated insulin sensitivity by insulin sensitivity index (ISI) differ
between low weight, normal weight, and obese healthy young men during a glucose clamp
experiment which is considered as 'gold standard' method in this context in diabetes
research [6]. A hyperinsulinemic-euglycemic clamp places plasma glucose concentration under the
investigator's control and thus breaks the endogenous glucose-insulin feedback loop.
The technique consists of an insulin infusion at predetermined fixed dosage and a
variable glucose infusion rate. Under steady-state conditions of euglycemia, the glucose
infusion rate equals glucose uptake by all tissues in the body [6] and is therefore a measure of glucose tolerance [7].

Methods

We studied 15 low weight (BMI between 17.6 and 19.8 kg/m2; mean 18.97 ± 0.20 kg/m2), 15 normal weight (BMI between 20.2 and 23.5 kg/m2; mean 21.86 ± 0.30 kg/m2), and 15 obese (BMI between 30.1 and 43.6 kg/m2; mean 33.81 ± 1.10 kg/m2) male volunteers aged 18-30 years. All subjects had a regular sleep-wake cycle during
the week before the experiments. Exclusion criteria were acute or chronic illness,
diabetes in 1-degree family members, alcohol or drug abuse, smoking, competitive sports,
shift work, exceptional physical or mental stress, and any kind of current medication.
The study conformed to the Declaration of Helsinki and was approved by the Ethics
Committee on Research Involving Humans of the University of Luebeck. All subjects
gave written informed consent.

After an overnight fast of 12 hours, subjects arrived at the research unit at 8:00
a.m. One cannula was inserted into a peripheral vein on the back of the hand and a
second cannula into an antecubital vein of the contralateral arm. The first blood
sample was taken at 8:30 a.m. to determine baseline concentrations of circulating
glucose and insulin. At 8:40 a.m., the glucose clamp was started by administration
of an insulin bolus (H-insulin, Hoechst, Frankfurt, Germany) of 5 mU/kg body weight
(BW)/min over 2 min. Thereafter, insulin was infused at a constant rate of 1.5 mU/kgBW/min
until the end of the clamp. A 20% glucose solution was infused simultaneously at a
variable rate to maintain plasma glucose values at approximately 5.5 mmol/l for 45
minutes. Thereafter, insulin infusion was stopped, and plasma glucose concentrations
were normalized by increasing glucose infusion rates.

Blood samples were drawn at 5-minute intervals to measure plasma glucose concentrations
(B-Glucose-Data-Management, HemoCue GmbH, Grossostheim, Germany) and every 10 minutes
for the determination of hormones. Blood was centrifuged within 5 min after withdrawal
and serum as well as plasma was kept at -72°C until assay.

Values are presented as mean ± SEM. To test for the criteria required for multivariate
analyses, we performed a Kolmogorov-Smirnov test on each variable. A Kolmogorov-Smirnov
P-value of .05 or greater was considered as a normal distribution All continuous variables
were found normally distributed in each group (all Kolmogorov-Smirnov Z > 0.432, all P > 0.500). Thus, statistical analyses were based on analysis of variance for repeated
measurements (ANOVA) with the factors "time" (for repeated measurements during the
experiment) and "group" (for differences between normal, low weight, and obese participants).
For pairwise comparisons between groups, unpaired Student's t-test was used. ISI was
calculated as described by Eriksson et al. [8] and respective P-values were revealed by one-way ANOVA. Correlation analyses were performed by bivariate
correlation analysis according to Pearson. A P-value less than 0.05 was considered significant.

Baseline serum insulin concentrations were significantly higher in obese as compared
with low and normal weight subjects (87.43 ± 8.95 vs. 42.88 ± 16.33 and 44.28 ± 12.14
pmol/l, respectively; all P < 0.027), while there was no difference in insulin values between the low and normal
weight groups (P = 0.946; small insert in Figure 1A). During the hyperinsulinemic-euglycemic clamp, serum insulin concentrations increased
in all groups with a significantly dampened increase in low and normal weight in comparison
to obese participants (447.40 ± 42.13 and 486.55 ± 33.45 vs. 790.96 ± 74.72 pmol/l;
all P < 0.001 for group effect; all P > 0.388 for group by time interaction) and comparable insulin values in the low and
normal weight group (P = 0.594 for group effect; P = 0.591 for group by time interaction). Calculated ISI values were significantly higher
in low and normal weight than in obese subjects (13.43 ± 1.57 and 17.82 ± 3.87 vs.
6.09 ± 1.09 mg*100*l*kg-1*min-1*mU-1; all P < 0.010) without any difference between the low and normal weight groups (P = 0.303).

Discussion

Our data demonstrate that moderate underweight is associated with impaired glucose
tolerance in healthy human volunteers. While a relationship between obesity and glucose
intolerance is well known, our findings suggest that generally all deviations from
normal BMI may be accompanied by an increased risk of developing T2DM in later life.
One could thus conclude that the maintenance of body weight within the normal range
has first priority in the prevention of this disease.

To our knowledge, this is the first study that shows significant associations between
low weight and IGT under metabolically well controlled conditions of a standardized
glucose clamp. In our study, glucose infusion rates required to maintain euglycemic
glucose values were significantly decreased in low weight as compared with normal
weight subjects but were comparable to those measured in obese participants. Moreover,
we found a positive relation between BMI and glucose tolerance in low weight and normal
weight subjects as well as a negative correlation in obese participants. To date,
little is known about the relationship between low weight and IGT in healthy subjects.
Former studies investigated glucose tolerance in patients with significant underweight
within the scope of a pathologic state. An impairment of glucose tolerance has previously
been seen in patients with cancer [4,9,10]. Additionally, IGT has also been found in middle-aged subjects with severe chronic
malnutrition [3] as well as in young women with anorexia nervosa [5]. Moreover, studies in older adults indicated an association between poor nutritional
status as well as hypomagnesaemia and risk of T2DM [11-13]. These findings are in line with animal studies showing that protein-energy restriction
decreases glucose-induced insulin secretion and worsens post-receptor signaling of
insulin [14]. Interestingly, such impairment in glucose-induced insulin release has also been
seen in lean middle-aged and older patients with T2DM [15] implicating that the here described impairment of glucose tolerance in low weight
young men could be a consequence of decreased insulin sensitivity in these subject.
In our study, however, insulin concentrations as well as ISI were well comparable
between low weight and normal weight participants indicating that BMI between 17.6
and 19.8 kg/m2 is not accompanied by changes in insulin sensitivity.

Conclusions

In summary, we found a close association between IGT and BMI equally in both low weight
and obese participants. This finding is of high clinical relevance because low weight
in adults has been neglected as risk factor for diabetes so far and the strong relationship
suggests that the prevention of obesity and low weight may be crucial to overcome
the increasing problem of T2DM. In consequence, clinicians should consider the fact
that all subjects with deviations from normal BMI values, regardless of their plasma
glucose levels, may display glucose intolerance and thereby could develop diabetes
in later life. Thus, also subjects with moderate underweight should be advised to
normalize their BMI for prevention.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

KMO designed the project and supervised the experiments. AS performed the experiments
and acquired data. KJC and AS provided suggestions on the project, data interpretation,
and the manuscript. KJC performed statistical analyses. KMO and KJC wrote the manuscript.
All authors read and approved the final draft.

Acknowledgements

We thank Kai-Uwe Duysen of Medical Department I for organizational work and technical
assistance. The study was supported by the German Research Foundation (Deutsche Forschungsgemeinschaft,
DFG, KFO 126).